This invention relates to relates to Schottky diode devices and more specifically relates to a novel termination for a Schottky diode.
Schottky diodes are well known devices in which a metal contact of high work function material, such as molybdenum, platinum, tungsten, palladium, titanium and their silicides are in direct contact with a silicon substrate, usually an N type epitaxially formed silicon layer formed on an N+ silicon substrate.
It is also known that a P+ guard ring diffusion should surround the periphery of the active Schottky contact area to permit higher breakdown voltage. The guard rings used have a high P+ concentration characterized in having a surface conductivity of about 180 ohms per square (a concentration of about 6E18 atoms/cm3) for a 30 to 45 volt Schottky.
It is desirable to use a thinner epitaxial layer and have a higher breakdown voltage if possible. It is known that a higher breakdown voltage can be obtained by using a reduced concentration guard ring which has a higher avalanche capability. However, when the guard ring concentration is reduced, the P/N junction efficiency in injecting minority carriers is low, resulting in a very high forward drop at high forward current.
In accordance with the invention, a reduced concentration guard ring is provided for a high voltage Schottky (for example, a 100 volt Schottky) and a shallow P+ region is added at the top of the guard ring to increase minority carrier injection at the P/N junction between the guard ring and epi at very high forward currents. As a result, the benefits of the lower concentration guard ring are retained and a thinner epi layer can be used for a given breakdown voltage.
By way of example, the guard ring diffusion may have a surface concentration producing a surface conductivity of about 1500 ohms per square (corresponding to a concentration of about 2E17 atoms/cm3) and a depth of about 2.5 microns. A shallow high concentration (P+) diffusion at the top of the guard ring may have a depth of about 0.5 microns and a concentration at the silicon surface of about 5E18 atoms/cm3.